The most common plant fiber is cotton, which is typically[3] spun into fine yarn for mechanical weaving or knitting into cloth.

Cotton and polyester are the most commonly spun fibers in the world. Cotton is grown throughout the world, harvested, ginned, and prepared for yarn spinning. Polyester is extruded from polymers derived from natural gas and oil. Synthetic fibers are generally extruded in continuous strands of gel-state materials. These strands are drawn (stretched), annealed (hardened), and cured to obtain properties desirable for later processing.

Synthetic fibers come in three basic forms: staple, tow, and filament. Staple is cut fibers, generally sold in lengths up to 120mm. Tow is a continuous "rope" of fibers consisting of many filaments loosely joined side-to-side. Filament is a continuous strand consisting of anything from 1 filament to many. Synthetic fiber is most often measured in a weight per linear measurement basis, along with cut length. Denier and Dtex are the most common weight to length measures. Cut-length only applies to staple fiber.

Filament extrusion is sometimes referred to as "spinning" but most people equate spinning with spun yarn production.

The most commonly spun animal fiber is wool harvested from sheep. For hand knitting and hobby knitting, thick, wool and acrylic yarns are frequently used.

Other natural fibers that can be used for yarn include linen[4] and cotton.[5] These tend to be much less elastic, and retain less warmth than the animal-hair yarns, though they can be stronger in some cases. The finished product will also look rather different from the woollen yarns. Other plant fibers which can be spun include bamboo, hemp, corn, nettle, and soy fiber.

A fully restored Derby Doubler, winding a sliver lap ready for finisher carding at Quarry Bank Mill in the UK.

In general, natural fibers tend to require more careful handling than synthetics because they can shrink, felt, stain, shed, fade, stretch, wrinkle, or be eaten by moths more readily, unless special treatments such as mercerization or superwashing are performed to strengthen, fix color, or otherwise enhance the fiber's own properties.

Protein yarns (i.e., hair, silk, feathers) may also be irritating to some people, causing contact dermatitis, hives, wheezing, or other reactions. Plant fibers tend to be better tolerated by people with sensitivities to the protein yarns, and allergists may suggest using them or synthetics instead to prevent symptoms. Some people find that they can tolerate organically grown and processed versions of protein fibers, possibly because organic processing standards preclude the use of chemicals that may irritate the skin.

When natural hair-type fibers are burned, they tend to singe and have a smell of burnt hair; this is because many, as human hair, are protein-derived. Cotton and viscose (rayon)yarns burn as a wick. Synthetic yarns generally tend to melt though some synthetics are inherently flame-retardant. Noting how an unidentified fiber strand burns and smells can assist in determining if it is natural or synthetic, and what the fiber content is.

Both synthetic and natural yarns can pill. Pilling is a function of fiber content, spinning method, twist, and fabric construction.

A Spinning Jenny, spinning machine which was significant in the beginning of the Industrial Revolution

S- and Z-twist yarn

Spun yarn is made by twisting staplefibres together to make a cohesive thread, or "single."[6] Twisting fibres into yarn in the process called spinning can be dated back to the Upper Paleolithic,[7] and yarn spinning was one of the very first processes to be industrialized. Spun yarns may contain a single type of fibre, or be a blend of various types. Combining synthetic fibres (which can have high strength, lustre, and fire retardant qualities) with natural fibres (which have good water absorbency and skin comforting qualities) is very common. The most widely used blends are cotton-polyester and wool-acrylic fibre blends. Blends of different natural fibres are common too, especially with more expensive fibres such as alpaca, angora and cashmere.

Yarn is selected for different textiles based on the characteristics of the yarn fibres, such as warmth (wool), light weight (cotton or rayon), durability (nylon is added to sock yarn, for example), or softness (cashmere, alpaca).

Yarn is composed of twisted strands of fiber, which are known as plies when grouped together.[8] These strands of yarn are twisted together (plied) in the opposite direction to make a thicker yarn. Depending on the direction of this final twist, the yarn will have either s‑twist (the threads appear to go "up" to the left) or z‑twist (to the right). For a single ply yarn, the direction of the final twist is the same as its original twist. The twist direction of yarn can affect the final properties of the fabric, and combined use of the two twist directions can nullify skewing in knitted fabric.[citation needed][9]

Filament yarn consists of filament fibres (very long continuous fibres) either twisted together or only grouped together. Thicker monofilaments are typically used for industrial purposes rather than fabric production or decoration. Silk is a natural filament, and synthetic filament yarns are used to produce silk-like effects.

Texturized yarns are made by a process of air texturizing filament yarns (sometimes referred to as taslanizing), which combines multiple filament yarns into a yarn with some of the characteristics of spun yarns.

A comparison of yarn weights (thicknesses): the top skein is aran weight, suitable for knitting a thick sweater or hat. The manufacturer's recommended knitting gauge appears on the label: 5 to 7 stitches per inch using size 4.5 to 5.1 mm needles. The bottom skein is sock weight, specifically for knitting socks. Recommended gauge: 8 to 10 stitches per inch, using size 3.6 to 4.2 mm needles.

Yarn drying after being dyed in the early American tradition, at Conner Prairie living history museum.

Yarn quantities for handcrafts are usually measured and sold by weight in ounces or grams. Common sizes include 25 g, 50 g, and 100 g skeins. Some companies also primarily measure in ounces with common sizes being three-ounce, four-ounce, six-ounce, and eight-ounce skeins. Textile measurements are taken at a standard temperature and humidity, because fibers can absorb moisture from the air. The actual length of the yarn contained in a ball or skein can vary due to the inherent heaviness of the fibre and the thickness of the strand; for instance, a 50 g skein of lace weight mohair may contain several hundred metres, while a 50 g skein of bulky wool may contain only 60 metres.

There are several thicknesses of craft yarn, also referred to as weight. This is not to be confused with the measurement and/or weight listed above. The Craft Yarn Council of America is making an effort to promote a standardized industry system for measuring this, numbering the weights from 1 (finest) to 6 (heaviest).[10] Some of the names for the various weights of yarn from finest to thickest are called lace, fingering, sport, double-knit (or DK), worsted, aran (or heavy worsted), bulky, and super-bulky. This naming convention is more descriptive than precise; fibre artists disagree about where on the continuum each lies, and the precise relationships between the sizes.

Another measurement of yarn weight, often used by weavers, is wraps per inch (WPI). The yarn is wrapped snugly around a ruler and the number of wraps that fit in an inch are counted.

Labels on yarn for handicrafts often include information on gauge, known in the UK as tension, which is a measurement of how many stitches and rows are produced per inch or per cm on a specified size of knitting needle or crochet hook. The proposed standardization uses a four-by-four inch/ten-by-ten cm knitted or crocheted square, with the resultant number of stitches across and rows high made by the suggested tools on the label to determine the gauge.

In Europe, textile engineers often use the unit tex, which is the weight in grams of a kilometre of yarn, or decitex, which is a finer measurement corresponding to the weight in grams of 10 km of yarn. Many other units have been used over time by different industries.